1. Field of the Invention
The present invention relates to an image pickup apparatus such as an electronic still camera and, more particularly, to a focus adjustment apparatus, a focus adjustment method, and a computer program which can be applied to the image pickup apparatus.
2. Description of the Related Art
In image pickup apparatuses such as electronic still camera, video camera, and the like in the related arts, a focus adjustment function called “auto focus” (hereinbelow, referred to as AF) is used in order to focus a photographing optical system on an object. In AF, such a system that image data is obtained from an image pickup element such as CCD, CMOS sensor, or the like and an in-focus position of a focus lens is calculated on the basis of focus evaluation values which are obtained from the image data is known. According to such a system, the position of the focus lens where the focus evaluation value appears to be a maximum value is set to the in-focus position. Therefore, in AF, the focus lens is driven in a predetermined range and the focus evaluation value of each picture (frame) which is obtained during such a drive period of time is stored in correspondence to the focus lens position. The position of the focus lens which is made to correspond to one picture (frame) is set as a position corresponding to half of a movement amount of the focus lens in an exposure period of one picture, that is, as an intermediate position between the position at a time point of start of exposure and the position at a time point of end of the exposure. The position of the focus lens is obtained for a blanking period of one picture and the intermediate position is calculated from an exposure time and a driving speed of the focus lens.
A method whereby a plurality of pixel signals of an image pickup element are read out while shifting a period, thereby increasing a reading speed as a whole is proposed. For example, according to Japanese Patent Application Laid-Open No. H06-245025, signals from odd-numbered pixels and even-numbered pixels among a plurality of pixels are output from a sensor chip with a half-period difference in time therebetween and, thereafter, are combined, thereby realizing a high reading speed. By using such a method, since a period at which the focus evaluation value is obtained is also shortened, many focus evaluation values can be obtained for a short time.
There is a case where even if the lens is controlled so as to be moved at a predetermined speed, an actual moving speed is not constant but fluctuates. In the case of calculating the focus lens position from only the time and the control speed as mentioned above, if the actual moving speed fluctuates during the driving of the focus lens, an error occurs between the calculated intermediate position and the actual intermediate position in an exposure period. Particularly, in the case where a stepping motor of open-loop control is used as a driving motor of the focus lens, since the actual position during the driving is not detected, even if the moving speed fluctuates, it cannot be known. Thus, an error is liable to occur in a calculation result.
It is, therefore, an aspect of the invention is to provide an image pickup apparatus having such a focus adjustment function that even if a moving speed of a focus lens fluctuates, by improving a calculation precision of the focus lens position as an obtaining position of a focus evaluation value, a focus adjustment precision can be improved.